In LTE (Long Term Evolution) Advanced in 3GPP (Third Generation Partnership Project), OFDMA (Orthogonal Frequency Division Multiplexing Access) using MU-MIMO (multi-user multiple-input multiple-output) has been proposed. In the downlink transmission of MU-MIMO, one base station communicates not only with multiple mobile terminals (UE, user equipment), but also can transmit different data streams (layers, ranks) to one mobile terminal simultaneously.
Recently, a heterogeneous network (sometimes abbreviated “HetNet”) is proposed in which multiple types of radio base stations (macro base stations, pico base stations, femto base stations, remote radio heads, etc.) having different transmission powers (radio capabilities), are deployed in a multi-layered way (for example, see Non-patent Document 1).
In a heterogeneous network, base stations having higher transmission powers (transmission capabilities), e.g., macro base stations are likely to be selected as the radio access points for mobile terminals 300 at the stage of cell search or handover in comparison with other base stations having lower transmission powers (transmission capabilities), e.g., pico base stations. Accordingly, it is assumed that connections of mobile terminals 300 are concentrated on base stations with higher transmission powers, and thus, there will be a tendency for excessive communication load at base stations with higher transmission powers.
Accordingly, a technique called cell range expansion has been proposed. The cell range expansion is a technique to give an offset value (bias value) to the reception quality or a reception power from a low-power base station, the reception quality or the reception power being an index for cell selection by the mobile terminal. The reception quality or the reception power from the low-power base station to which an offset value has been added (or added in the unit of dB) is compared with the reception quality or the reception power from the macro base station. As a result, the reception quality or the reception power from the low-power base station is likely to become better than the reception quality or the reception power from the macro base station. Consequently, since mobile terminals select to connect to the low-power base station than the macro base station, the cell range of the low-power base station is expanded, and it is likely that the communication load of the macro base station is reduced.
However, when the cell range of the low-power base station is expanded by the cell range expansion (CRE), the mobile terminal located at the edge of the cell of the low-power base station may be subject to high interference by radio waves from the neighboring macro base station. Therefore, a technique called enhanced inter-cell interference coordination or enhanced inter-cell interference control, which is an extension of the inter-cell interference coordination or inter-cell interference control, has been proposed. This technique is abbreviated as eICIC. The eICIC is described, for example, in Non-patent Document 2.
The eICIC is classified into a frequency domain-based eICIC and a time domain-based eICIC. In either type, the eICIC is a technique to limit resources available for a macro base station in order to prevent or minimize interference with mobile terminals connected with the low-power base station.
In the frequency domain-based eICIC, multiple frequency bands are prepared. A first frequency band is used for the downlink transmission from a macro base station to mobile terminals connected to the macro base station and the downlink transmission from a low-power base station to the mobile terminals at the center of the cell of the low-power base station (the mobile terminal which is connected to the low-power base station even without, for example, CRE). A second frequency band is used for the downlink transmission from a low-power base station to mobile terminals in the edge of the cell of the low-power base station (mobile terminals connected to the low-power base station due to, for example, CRE), but is not used for the downlink transmission from the macro base station. Thus, it is assumed that interference due to radio waves from the macro base station to mobile terminals at the edges of the cells of the low-power base stations is prevented.
In the time domain-based eICIC, the macro base station and the low-power base station use the same frequency band, but different time units (for example, subframes) are used for different purposes. FIG. 1 is a graph exemplifying time changes of downlink transmission powers of the macro base station and the low-power base station in the time domain-based eICIC. As will be apparent from FIG. 1, the low-power base station is capable of doing continuous downlink transmission. However, the macro base station can perform downlink transmission only intermittently. As a result, a period during which only low-power base stations perform the downlink transmission (protected subframe, PSF) and a period during which both macro base stations and low-power base stations perform downlink transmission (non-protected subframe, NSF) are generated. The non-protected subframes are used for downlink transmission from the macro base station to mobile terminals connected with the macro base station and for downlink transmission from the low-power base stations to mobile terminals at the centers of the cells of the low-power base stations (e.g., mobile terminals connected with the low-power base stations without CRE). The protected subframe is used for the downlink transmission from a low-power base station to mobile terminals in the edge of the cell of the low-power base station (mobile terminals connected to the low-power base station due to, for example, CRE). Thus, it is assumed that interference due to radio waves from the macro base station to mobile terminals at the edges of the cells of the low-power base stations is prevented.
To improve transmission efficiency from the macro base station, a modification of eICIC is proposed in which the downlink transmission from the macro base station in a specified resource group is not halted, but the downlink transmission from the macro base station in a specified resource group (for example, protected subframe) is executed at a low transmission power (Non-patent Document 3). FIG. 2 is a graph exemplifying time changes of a downlink transmission power of the macro base station and the low-power base station in a modification of the time domain-based eICIC. In this case, although the transmission power from the macro base station is reduced in a specified resource group (protected subframe in FIG. 2), data transmission from the macro base station is allowed. Such a resource group in which the transmission power is reduced can be used for radio transmission to mobile terminals geographically close to the macro base station. Since the transmission power in the macro base station is reduced, even if the low-power base stations use these specified resource groups for mobile terminals connected to the low-power base station, it is likely that interference with the mobile terminals will be small.